Evaluating backpack capacity in the real world

…aka the most exciting topic ever.

It’s important to get the right size pack.  For evidence, I refer to the above photos.  Having to strap stuff outside is occasionally necessary, but it’s almost always bad style, and often somewhat hazardous (lost water bottles, paddle blades, etc).  Evaluating how big a pack will actually be once in use is crucial when shopping sight unseen, but is not as simple as it might seem, both because of the varying methods makers use to rate their bags, and because the shape and layout of a backpack heavily influences both how it measures out, and how usable that capacity becomes.  Due to a whole host of design issues, these last two are not infrequently in conflict.

Screen Shot 2017-03-17 at 10.12.32 PM

Backpack volume is measured in cubic liters or inches.  I strongly prefer the former, as I want to help the standard system finally release its imperialist claws, but mostly because two digit numbers are simpler.  In any case, memorizing a few mental benchmarks to quickly convert the two is useful.  30 liters is not quite 2,000 cubic inches, which is good a day ski touring in frigid temps, or 4 days out in the height of summer.  50 liters is 3,000 cubic inches, which is a 10 day UL backpacking load, or a packrafting day load.  90 liters is 5,500 cubic inches, which is a week long fall hunting load.  130 liters is 8,000 cubic inches, which is not too big for a 10 day packrafting expedition.  And so forth.

The “industry standard” set by the American Society for Testing and Materials is to fill the pack, and all pockets which close with a zipper, with 20mm diameter plastic spheres and then measure the space those balls occupy in another container.  The problems this method presents are diverse and of consequence.  For one, bog standard pingpong balls are not exactly 20mm, so a company aspiring to this standard must purchase special balls.  And if they plan to make a 130 liter pack, quite a few of them.  For another, including zippered pockets but excluding pockets that close with straps or drawstrings, is biased against certain classes of backpacks, if maximum size for a given weight is the goal.  For a third, most pockets, zippered or not, are not designed so that they retain their volume well when the main bag is stuffed full.  Most severely, 20mm balls are rather large, and in my experience significantly under-report size for bags and especially pockets with more complex curves and shaping.

The end result is that pack makers, especially smaller pack makers, usually just make a number up.  They draw on past experience, maybe look at numbers from other companies, make the ethical and stylistic decision concerning how conservative they want their numbers to be, and then go for it.  Some manufacturers follow the old method grading trade climbs, and pick the lowest number they can say out load with a straight face.  A lot more do the same with the highest number that will stick.  Obviously, factors like the personal biases and breadth of experience (or lack thereof) of the person or people in question is of enormous consequence, as is the market in which they seek to operate.  If the intended competition is all 50 liters, it makes sense to call your bag a 50 liter one, too.

For example, look at the Stone Glacier R3 (tactical Solo) on the left, and the Exo K2 2000 daypack on the right.  The R3 is rated as 54 liters, the 2000 as 46 (32 liters for the main bag, 14 for the rolltop and stretch pocket).  I didn’t tape either at the Hunt Expo, though I have owned and used a Solo.  The R3 might be a hair bigger, but overall the packs struck me as very similar in size.  Stone Glacier with, in my opinion, a dead on assessment, Exo with a fairly conservative one.  Exo undersells by labeling their pack the 2000, and marketing it as a daypack, while the Solo/R3 is Stone Glacier’s “minimalist multi-day backcountry backpack.”  Exo made the choice to never give anyone the reasonable option to complain about buying a too small pack, while Stone Glacier appeals to the hard core and aspirational in equal measure.  Neither post actual dimensions of their website, though that’s likely because few customers are geeky enough to want them.  Both made a volume determination based not just on fact, but on marketing.

One company who does post numbers, and very detailed ones, is Hyperlite Mountain Gear.  Luke and I had the chance to see the whole HMG line at IME last weekend, and while I once again failed to bring my tape measure, all HMG’s claimed dimensions seemed at the time quite accurate.  In fact, I’d say that their packs are the current benchmark against which other claims ought to be, and can be most easily and accurately be, measured.  This is helped by a simple bag shape, shared dimensions within families (e.g. a 3400 Porter and Windrider have identical main bags), and in the case of the Windriders anemic pockets with little real capacity worth mentioning.

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HMG’s method of stating height, upper and lower circumference, and back width is the one I find most succinct and demonstrative.  Not coincidentally, it is most commonly found among cottage UL backpack makers, though Osprey at least nods in this direction with most of their products.  It gives a prospective buyer a concrete sense of both overall size, and where that size is being used.  Take some conservative numbers from the HMG 4400 bags for example; with a closed height of 35 inches, and an average overall circumference of 41 inches, a simple cylinder volume will be 4645 cubic inches, or about 76 liters.  Cutting a bit off that due to the pack not being a cylinder when in use adjusts things nicely to 70 real liters.  If your 70 liter pack has dimensions which add to a lot less than an HMG 4400, something suspicious is afoot.

HMG is a good and easy example to follow because all their bags are fairly simple cylinders, with a steady bottom to top taper and little other shaping.  The math will be for them simpler and more reliable than any other, save an orthotope.  Bag shapes that smooth corners, and have a narrower and wider profile in some areas compared to others, will map out as having less capacity.  Depending on what you’re packing, they will have less actual capacity, too.  Loose things like sleeping bags and clothing, and to a less extent tents, sleeping pads, and food, can mold to fit curves and corners if they aren’t packed to tightly beforehand.  Other items, like bear canisters or a rolled packraft, are rigid and cannot adapt.  The closer a pack bag is to a square or fat rectangle in cross section, the more forgiving it will be to pack, and the better it will measure out on paper.


The above canyon selfie was taken while carrying a tall and fairly thin (2:1 width/depth ratio in spots) backpack.  Such a bag offers more limited options for bulky and semi-rigid objects, but is desirable anyway because it places weight close to the user, and fits easily through tight places.  Design is a tug of war between myriad factors, with no free lunch.


My first takeaway from all of this is to be a skeptical consumer.  Get the dimensions of a pack you can’t see first hand, ideally from both the manufacturer and a third party.  If the numbers don’t seem to add up, then they probably don’t  The only way to make an 80 liter pack is to have the height, width and depth add up to that volume.

My second takeaway is to think of volume and dimensions as intertwined factors, when one moves so to do the others.  If you want a shorter pack to, for example sneak through Oak Brush, you’ll either have to make do with less space, or have a deeper and/or wider pack.  For example, if your 80 liter pack is only 30 inches tall, you’ll need the average circumference to be almost 48 inches, which is quite a lot.

Lastly, while there is a lot to be said for buying a pack that will enforce packing discipline, and a lot to be said for having a small pack, generally, don’t be too much of a slave to theory.  Fabric doesn’t add much weight, and be able to put everything in the bag, without egregious cramming, is almost always better for you and all your gear.


One thought on “Evaluating backpack capacity in the real world

  1. Pingback: March 18, 2017 – The Bushcraft Report ™

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